1. Field of the Invention
The present invention relates to a slot antenna apparatus, and more particularly, to a slot antenna apparatus which is operated as a circular polarization antenna.
2. Description of the Related Art
A slot antenna apparatus comprises a conductor member composed of a metal foil or a metal plate in which a slot having a predetermined size is formed and a circuit substrate in which a high-frequency circuit including an amplifier, an oscillator or the like is arranged. By feeding a power to an appropriate location of the conductor member through a feeding means such as a feeding line connected to the circuit substrate, a slot is excited and a linearly polarized wave can be irradiated. As a conventional example of such a slot antenna apparatus, an antenna apparatus in which an antenna unit is provided at one side of the circuit substrate is widely known (for example, see Japanese unexamined Patent Application Publication No. 2003-234615 (Pages 3 to 4, FIG. 3)).
FIG. 6 is a plan view of essential elements of a conventional slot antenna apparatus and shows an antenna unit 2 provided at one side of a substrate 1 composed of a dielectric. In FIG. 6, on one surface of the substrate 1, a conductor layer 4 having a slot 3 is formed, and on the other surface of the substrate 1, a feeding line 5 traversing the slot 3 in a width direction is formed. In addition, the extending portion (not shown) of the substrate 1 forms a circuit unit 6 in which a high-frequency circuit is arranged. The conductor layer 4 or the feeding line 5 is formed by patterning copper foil or the like and a rectangular opening surrounded by the conductor layer 4 forms the slot 3. The length of the slot 3 is set to about half of the resonance length λ. The feeding line 5 is a micro-strip line connected to the circuit unit 6. By feeding the power to both ends of the slot 3 in a width direction through the feeding line 5, the slot 3 can be excited.
However, the conventional slot antenna apparatus is generally designed as the linear polarization antenna. But, when an additional slot extending in a direction perpendicular to a longitudinal direction of the slot 3 is formed in the vicinity of the slot 3 and is excited with a phase difference of about 90 degrees with respect to a resonance mode of the slot 3 in FIG. 6, a circular polarization slot antenna apparatus is obtained. However, since the conventional antenna uses the extending portion (one side) of the substrate 1 in which the high-frequency circuit is arranged as a region of an antenna unit 2, when a pair of circular polarization slots and a pair of feeding lines are formed in the antenna unit 2, a wide space is required in the antenna unit 2 so that an area of the substrate 1 increases. As a result, the miniaturization of the overall apparatus cannot be accomplished. Further, in order to generate the phase difference of about 90 degrees in the resonance modes of the pair of slots, the phase difference circuit unit of 90 degrees must be provided in the feeding circuit. This results in the circuit structure becoming complex and the cost increasing.
Moreover, in order to minimize the slot antenna apparatus, the structure that the high-frequency circuit, the feeding line and the conductor layer having the slot are provided in each layer of the multilayered substrate can be considered. However, if the multilayered substrate is employed, the cost thereof increases greatly because the manufacturing process thereof is complex.